When writing is performed by a mechanical pencil, the writing is often performed in a state where a tubular shaft of the mechanical pencil is slightly inclined with respect to a surface of a sheet of paper. If the writing is continued in the state where the tubular shaft of the mechanical pencil is inclined in this way, the tip end of a writing lead is unsymmetrically worn, and an area of a contact surface of the tip end of the writing lead which contacts the surface of the sheet of paper, namely, an area of the unsymmetrically worn surface of the tip end of the writing lead is increased. Consequently, a phenomenon occurs in which lines that are drawn on the surface of the sheet of paper after the area of the unsymmetrically worn surface is increased will become thick as compared to lines which were drawn on the surface of the sheet of paper when the writing was commenced. Moreover, a phenomenon occurs in which the increase in the area of the unsymmetrically worn surface of the tip end of the writing lead will produce blurred areas in the drawn lines.
In order to solve these problems, there has been proposed a writing lead rotating mechanism which allows a chuck unit, disposed in a tubular shaft of a mechanical pencil and always biased forward, and a writing lead held by the chuck unit to be rotated in a circumferential direction, as the chuck unit is moved rearward by a writing force produced by pushing the writing lead against the surface of the sheet of paper, or the chuck unit is moved forward when the writing lead is released from the writing force (refer to Patent Literature 1).
The writing lead rotating mechanism of the mechanical pencil described in the Patent Literature 1 includes a rotary element rotatably disposed in the tubular shaft and adapted to be rotated together with the chuck unit, the rotary element having a first cam face formed at a rear end thereof and a second cam face formed at a tip end thereof, a first stationary cam face provided in the tubular shaft and disposed on a rear end side of the first cam face so as to face the first cam face, and a second stationary cam face provided in the tubular shaft and disposed on a tip end side of the second cam face so as to face the second cam face. As the chuck unit is moved rearward in the tubular shaft by application of the writing force to the writing lead, the first cam face of the rotary cam element is operatively engaged with the first stationary cam face, whereby the rotary cam element is rotated at a fixed rotational angle in the circumferential direction. Thereby, the writing lead held by the chuck unit is rotated at the fixed rotational angle in the circumferential direction. Moreover, when the chuck unit is moved forward in the tubular shaft by release of the writing force applied to the writing lead, the second cam face of the rotary cam element is operatively engaged with the second stationary cam face, whereby the rotary cam element is rotated at the fixed rotational angle in the circumferential direction. Thereby, the writing lead held by the chuck unit is rotated at the fixed rotational angle in the circumferential direction. By causing the writing lead to be rotated using the writing force in this way, it is possible to suppress an increase in an area of an unsymmetrical wearing surface of a tip end of the writing lead.